Add a downloads page to hbase.apache.org to tie mirrored artifacts to their hash and signature

I tried to push to announce for 2.0.0 but it got blocked because mirrors don't have signatures and hashes (I only just noticed. Its probably been this way for ages....). Signatures and hashes are hosted on www.apache.org only. So, there needs to be a connection made between hashes and signatures hosted on apache.org and the associated artifacts gotten from mirrors. Projects usually do this on a 'download' page. Add one with 2.0.0 as first entry.

misty FYI